The present invention relates to a sliding bearing for internal combustion engines, the sliding bearing consisting of a pair of semi-cylindrical bearings which are assembled to each other so as to support a crankshaft.
Conventional sliding bearings for crankshafts have been fabricated by mating two semi-cylindrical bearings with each other so as to form a cylindrical configuration. A circumferential oil groove is provided on an inner surface of at least one of the pair of semi-cylindrical bearings, and lubricant oil is fed to an outer surface of a crankpin through the circumferential oil groove. In general, the circumferential oil groove has a constant depth (see JP-A-8-277831).
On the other hand, in order to reduce the quantity of a lubricant oil leakage from ends of the sliding bearing according to downsizing of oil pumps for supplying a lubricant oil, a particular sliding bearing has been used in recent years, in which sliding bearing a constricted portion is provided to a circumferential oil groove so as to decrease the cross-sectional area of the circumferential oil groove toward the ends of the semi-cylindrical bearings of the sliding bearing from a circumferential center region of the respective semi-cylindrical bearing, or alternatively an area reduction portion is provided to a circumferential oil groove by forming protrusions on a bottom of the circumferential oil groove between circumferential ends and an oil hole for lubricant-oil supply provided to the semi-cylindrical bearings, whereby enabling a lubricant oil supply pressure to decrease (see JP-A-4-219521, JP-A-2005-76755 and JP-A-2006-144913).
With regard to a lubricant oil supply to sliding bearings used in an internal combustion engine, the lubricant oil is fed from an outside of a main sliding bearing for a crankshaft to a circumferential oil groove formed on an inner surface of the main sliding bearing, to an inner sliding surface of the main sliding bearing, and to an inner sliding surface of a sliding bearing for a crankpin of the crankshaft.
When an internal combustion engine is run in, foreign substances remaining in a lubricant oil passage are apt to be mixed in the lubricant oil being supplied to the circumferential oil groove of the main sliding bearing. Herein the foreign substances mean metal chips produced during machining to form an oil passage in an engine block, a molding sand used when casting the engine block, and so on. The foreign substances are entrained by a lubricant oil flow upon rotation of the crankshaft and discharged together with the lubricant oil through clearances, such as portions of chamfers, so-called crash relieves, etc., formed at bearing circumferential ends in conventional sliding bearings for internal combustion engines. In internal combustion engines in recent years, however, an advance-movement inertial force (i.e. an advancement inertial force causing foreign substances to go straight in a circumferential direction) acting on the foreign substances, which are accompanied in the lubricant oil flow and have a larger specific gravity than the lubricant oil, has increased in magnitude due to a trend of a high rotational speed of the crankshaft, so that the foreign substances are not discharged from clearance portions at mating ends of a pair of semi-cylindrical bearings constituting the sliding bearing but caused to move into a bearing sliding surface region of the other semi-cylindrical bearing without an oil groove on the inner surface thereof, whereby the bearing sliding surface of the sliding bearing have been liable to be damaged by the foreign substances.
On the other hand, in order to reduce the quantity of a lubricant oil leak from the ends of the pair of the semi-cylindrical bearings, there are proposed sliding bearings, in which a constricted portion is provided to an oil groove at a circumferential end of a semi-cylindrical bearing (see JP-A-4-219521, JP-A-2005-76755, and JP-A-2006-144913). In evaluating these sliding bearings from the viewpoint of the foreign substances, there is a problem that a flowing rate of the lubricant oil increases in a downstream side of the constricted portion in a flow direction of the lubricant oil whereby the inertial force acting on foreign substances entrained in the lubricant oil flow further increases in magnitude resulting in an increase in occasion when the foreign substances move into a bearing sliding surface region.